The present invention relates to a non-contact optical measuring device for measuring thickness or step height of a workpiece.
FIG. 3 shows a conventional optical measuring device using deviation measuring means disclosed in, for example, Japanese Patent Publication No. 10561/1981 applied to a measurement of thickness of a workpiece. In the same figure, on both sides of a workpiece 5, a pair of deviation measuring means 16 are disposed and outputs of the deviation measuring means 16 are supplied to a signal processing means 17.
Each deviation measuring means 16 includes a pulse generating circuit 1 for determining a driving timing of a semiconductor laser 3 and sampling times of sample-hold circuits 13 and 14 to be described later. A drive circuit 2 drives the semiconductor laser 3 on a basis of pulses generated by the pulse generating circuit 1. A lens 4 condenses lights from the semiconductor laser 3 to focus it on the workpiece 5 as a spot 6. A light receiving lens 7 focuses an image of the light spot 6 on the workpiece 6 onto a light receiving element 8 to convert a position of the image of the light spot 6 focussed thereby into an electric signal.
Reference numerals 9 and 10 are amplifiers for amplifying electric signals from the light receiving element 8, 11 is a subtractor for obtaining a difference between outputs of the amplifiers 9 and 10, 12 is an adder for adding the outputs of the amplifiers 9 and 10, 13 and 14 are sample-hold circuits for sampling and holding outputs of the subtractor 11 and the adder 12 on the basis of sample timing signals from the pulse generator 1, respectively, and 15 is a divider for obtaining a ratio of the outputs of the sample-hold circuits 13 and 14. The signal processing means 17 functions to add a pair of outputs from the deviation measuring means 16 from which a thickness of the workpiece 5 is calculated.
In operation, the semiconductor laser 3 is driven by the drive circuit 2 such that it emits a pulsed beam. Light from the semiconductor laser 3 is condensed by the lens 4 and directed to a surface of the workpiece 5 perpendicularly. Any surface which is not an ideal mirror surface scatters an incident light and therefore it is possible to detect a bright light spot at various angles with respect to the incident light. When the lens 7 is disposed on the optical axis which makes a predetermined angle with respect to the irradiating beam to form an image of the light spot on the light receiving plane of the light receiving elements 8, the output currents i1 and i2 of the light receiving element 8 become correspondent to the position of the light spot of the light receiving planes. These outputs are amplified by the amplifiers 9 and 10 and then signals corresponding to (i1-i2) and (i1+i2) are derived therefrom by the subtractor 11 and the adder 12 which are supplied to the sample-hold circuits 13 and 14, respectively. The latter circuits function to sample the input signals in synchronism with the drive pulse from the pulse generator 1 and the received light signal in the form of pulse is converted into d.c. signals. The calculation of (i1-i2)/(i1+i2) is performed by the divider 15 to obtain a signal proportional to the position of the light spot formed on the light receiving plane of the light receiving element 8, from which a deviation of the workpiece can be known.
In more detail, in FIG. 4 which shows the light receiving element 8 in FIG. 3 in detail, the light receiving element 8 includes a frame 31, an N type semiconductor 32 disposed in the frame 31, a P type semiconductor 33 formed on a surface of the N type semiconductor 32, an electrode 34 attached to the N type semiconductor 32, a first electrode 35 attached to the P type semiconductor 33, a second electrode 36 attached to the P type semiconductor 33, a power source 37, a resistor as load 38 and a resistor as load 39. 40 depicts an incident light reflected from the light spot 6 focussed by the light receiving lens 7 (cf. FIG. 3) and 41 is an intensity distribution P(x) of the incident light 40.
It is usual that a resistance of the P type semi-conductor 33 on the surface of the N type semiconductor 32 is relatively large and uniform. When the incident light 40 has an intensity distribution P(x) with respect to a distance x from a center of the light receiving element 8, currents i1 and i2 flowing through the small resistors 38, 39 can be approximated by the following equations (1) and (2). ##EQU1## where K1, K2 are constants and l is a distance from the center of the light receiving element 8 to an end thereof.
Therefore, by calculating a ratio of (i1-i2) to (i1+i2), it is possible to obtain a position of the center of incident light on the light receiving element 8. For example, when it is assumed that the resistance value of the P type semiconductor 33 is large, that resistance values of the N type semiconductor 32 and the resistors 38 and 39 are negligible compared with the resistance value of the P type semiconductor 33, respectively, and that the light spot can be considered as a point, the following equations are established: EQU r(l-x)i1=E (3) EQU r(l-x)i2=E (4)
where r is a resistance value per unit distance of the P type semiconductor in l direction.
From the equations (3) and (4), the following equation is obtained: ##EQU2##
When the outputs of the pair of deviation measuring means 16 obtained in this manner are represented by l1 and l2, respectively, and an initially set reference value is represented by K, the thickness T is calculated by the signal processing means 17 according to the following equation. EQU K-(l1+l2)=T (6)
Incidentally, when the relation between l1 and l2 is as shown in FIG. 6, a step height T is calculated by the following equation EQU (l1-l2)=T (7)
Since the conventional optical measuring device is constituted as above, when a workpiece to be measured is semitransparent, an irradiating light from one of the deviation measuring means passes partially through the workpiece or a reflection light from the workpiece falls in a light receiving element of the other deviation measuring means. Therefore, an error may be introduced into a measured value of thickness or step height.